# decay-copy

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< cpp‎ | standard library

template< class T > typename std::decay<T>::type decay-copy( T&& value ); |  | (since C++11) (until C++20) (exposition only*)
template< class T > requires std::convertible_to<T, std::decay_t<T>> constexpr std::decay_t<T> decay-copy( T&& value ) noexcept(std::is_nothrow_convertible_v<T, std::decay_t<T>>); |  | (since C++20) (exposition only*)

Returns std::forward<T>(value) (implicitly converted to the decayed type), a decayed prvalue copy of value.

### Parameters

value | - | the value to be copied

### Return value

A decayed copy of value as a prvalue.

### Notes

`decay-copy` was introduced by the resolution of LWG issue 929. It is initially used in the concurrency support library to ensure that arguments are decayed when passing-by-value, and is later used in the ranges library.

The language feature `auto`(x) introduced in C++23 also allows decayed copies to be created as prvalues. The only difference is that `decay-copy` always materializes value and produces a copy, while auto(expr) is a no-op if expr is a prvalue.

All usages of `decay-copy` in the standard library (see below) except `views::all`, `ranges::take_view` and `ranges::drop_view` are replaced with auto(x) since C++23.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3724 | C++20 | `decay-copy` was not constrained | constrained

### See also

(constructor) | constructs new `thread` object (public member function of `std::thread`)
(constructor) | constructs new `jthread` object (public member function of `std::jthread`)
async (C++11) | runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template)
ranges::begin (C++20) | returns an iterator to the beginning of a range (customization point object)
ranges::end (C++20) | returns a sentinel indicating the end of a range (customization point object)
ranges::rbegin (C++20) | returns a reverse iterator to a range (customization point object)
ranges::rend (C++20) | returns a reverse end iterator to a range (customization point object)
ranges::size (C++20) | returns an integer equal to the size of a range (customization point object)
ranges::data (C++20) | obtains a pointer to the beginning of a contiguous range (customization point object)
views::all_tviews::all (C++20) | a `view` that includes all elements of a `range` (alias template) (range adaptor object)
ranges::take_viewviews::take (C++20) | a `view` consisting of the first N elements of another `view` (class template) (range adaptor object)
ranges::drop_viewviews::drop (C++20) | a `view` consisting of elements of another `view`, skipping the first N elements (class template) (range adaptor object)

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